Satellite broadcast systems are used to distribute audio, video, and data content to end users using a constellation of communication satellites. Media content is encoded on modulated radio frequency signals that are transmitted to the satellite network in an uplink transmission. The communication satellites are equipped with multiple transponders that process the incoming uplink signal and then transmit a frequency-shifted version of the uplink signal as the downlink signal. Satellite broadcast systems thereby allow for wirelessly distributing media content without a direct line of sight between the uplink station and each subscriber. As these systems have evolved, there has been an ongoing demand for additional bandwidth and/or more efficient use of existing bandwidth to carry an ever-increasing amount of content. Due to a combination of existing legacy hardware systems and regulatory issues, most content continues to be broadcast over a set of transmission channels with fixed bandwidth capacities.
In a simple version, each of the transponders may function as repeaters that emit a frequency-shifted version of a portion of the uplink signal. Together, a set of transponders on the satellite can be used to create a downlink signal on a set of carriers that each correspond to one of the transponders. Thus, the set of transponders on a satellite define a set of downlink carriers (channels) for reception at receiver stations on the ground for each subscriber. The receiver stations include an antenna (e.g., a satellite dish) for detecting the downlink signals and a set top box (STB) that receives signals detected at the antenna and extracts the digital media content streams from those signals. The STB can include various hardware components for tuning and demodulating the radio frequency downlink signals, signal processing electronics for decoding, applying error correction, and/or video drivers that convert the media data stream into instructions for driving a video display.
To ease bandwidth constraints, video data is generally compressed prior to transmission using techniques based on standards such as MPEG-2 and MPEG-4 and various other coder/decoder specifications (codecs). Video compression reduces the required bandwidth of a given video stream while maintaining high visual quality. Compression also results in a variable bit-rate data stream. In particular, video compression requires a relatively high bit rate to represent video scenes depicting rapid movement or scenes with high degree of textured scenery (e.g., a sporting event) whereas a relatively low bit rate is required to represent video scenes depicting a stable background scene with slow movement in the foreground (e.g., a news program).
To accommodate a variable bit-rate compressed video signal on a fixed bit-rate carrier channel, the bandwidth capacity of the carrier channel must exceed the peak bit-rate of the video signal. For most types of compressed video content, the peak bit-rate exceeds the average bit-rate by a significant amount. To make efficient use of the bandwidth capacity of the carrier channels, multiple compressed video signals may be multiplexed together using a statistical multiplexer prior to transmission over the fixed bit-rate carrier channels. Because the bit-rate peaks of the different compressed video signals are unlikely to occur simultaneously, the combined signal provides a data stream with a relatively more stable bit-rate to be transmitted over the fixed bit-rate carrier. The combined signal may have a bit-rate that corresponds to the bandwidth capacity of a single transponder carrier, and the combined signal can be transmitted using a single transponder.
In practice, additional bandwidth margin is required for error correction, overhead messaging, and accounting for occasional bit-rate peaks in the combined signal, but the statistical multiplexing process still provides spectral efficiency benefits when transmitting multiple variable bit-rate signals over fixed bit-rate channels, such as satellite transponder carriers.
In addition to the compression used, the bandwidth required to stream video data also depends on the resolution and frame rate of the underlying video. A digital video stream can be thought of as a series of images of a particular resolution as well as associated audio content. Each frame of a video is an image with pixels arranged in rows and columns. Video resolution is typically characterized by the number of rows of pixels in each frame image. The frame rate typically refers to the speed at which each frame is displayed as the video is played out. The frame rate may also be referred to as the refresh rate (e.g., the time between successive updates of a given pixel value). Progressive video updates each pixel of a frame at each frame refresh. Interlaced video updates alternating rows for each frame refresh. Thus, interlaced video requires less bandwidth than progressive video for a given resolution and frame rate of the same video content.
For example, standard definition (SD) video refers to a class of video content with frames having 480 rows that are progressively scanned and frame rates of about 24 or 30 frames per second. For shorthand, such SD video may be identified as 480p24 (i.e., 480 rows per frame, progressively scanned, at 24 frames per second) or 480p30 (i.e., 480 rows per frame, progressively scanned, at 30 frames per second). Similarly, high definition (HD) video may be identified as 720p24 (i.e., 720 rows per frame, progressively scanned, at 24 frames per second) or 1080i30 (i.e., 1080 rows per frame, interlaced, at 30 frames per second), or 1080p24 (i.e., 1080 rows per frame, progressively scanned, at 24 frames per second).
Existing satellite transponders have bandwidth capacities sufficient to carry a multiplexed stream of a combination of two to ten compressed video streams when the underlying video content is SD video or HD video (e.g., 480p24, 1080i30 or 720p24). Such video content is referred to as low bandwidth, because the bandwidth requirement of such content is low compared to the bandwidth capacity of an individual satellite transponder carrier. However, video content requiring higher bit rates is being introduced, such as 1080p60, high definition three-dimensional video (which requires twice the frame rate to separately display content for perception by the left and right eyes in alternating frames), and 4 KTV, which has twice the number of rows as high definition television, and thus four times the number of pixels per frame (e.g., 2160p24). Such video content is referred to as high bandwidth, because the bandwidth requirement of such content is high compared to the bandwidth capacity of an individual satellite transponder carrier channel.